Preheat your oven to 350°F (175°C) and prepare a 9x13-inch baking dish by greasing it lightly.
In a separate bowl, combine the flour, cocoa powder, baking powder, and salt, whisking until well mixed.
In a large mixing bowl, beat the softened butter with both sugars until the mixture is fluffy and pale.
Incorporate the eggs one at a time, ensuring each is fully blended before adding the next, then mix in the vanilla extract.
Gradually fold the dry ingredients into the wet mixture until just combined, then gently stir in the chocolate chips.
Pour the brownie batter into the prepared pan, smoothing it out evenly.
Take spoonfuls of the cookie dough and drop them across the surface of the brownie batter.
With a knife or spatula, swirl the cookie dough into the brownie mix to create a beautiful marbled pattern.
Bake for 25-30 minutes, checking for doneness with a toothpick; it should come out with a few moist crumbs attached.
Allow the dessert to cool in the pan for about 15 minutes before slicing into squares for serving.